Prevention of crawling of metal oxide hollow articles along the support mandrel during sintering

ABSTRACT

The method of sintering amorphous of amorphous metal oxide, and in a preferred embodiment crucibles and tubing of fused silicon dioxide, by depositing a porous layer of amorphous metal oxide on a deposition surface, and thereafter partially sintering the edges of the metal oxide article to less than 90 percent of its theoretical density and substantially sintering the remainder of the metal oxide article to greater than 90 percent of its theoretical density. By only partially sintering the edge portions sufficient frictional forces are produced with the mandrel to exceed the surface tension forces created in the remainder of the metal oxide article by the higher sintering temperature thus preventing longitudinal contraction and crawling encountered in the prior art processes. For a silicon dioxide article, the edges of the mandrel are sintered at a temperature in the range of 1,250* C. to 1,350* C.; the remainder of the article is sintered at a temperature in the range of 1,450* C. to 1,550* C. The partially sintered portion is then severed from the fully sintered portion to produce the desired article.

United States Patent [72] Inventor Foster Lee Gray Dallas, Tex.

[21] Appl. No. 886,253

[22] Filed Dec. 18, 1969 [45] Patented Nov. 9, 1971 [73] Assignee Texas Instruments Incorporated Dallas, Tex.

[54] PREVENTION OF CRAWLING OF METAL OXIDE HOLLOW ARTICLES ALONG THE SUPPORT MANDREL DURING SINTERING 17 Claims, 9 Drawing Figs.

[52] U.S. Cl 264/60, 264/57, 264/64, 264/66, 264/67, 264/332 [51] Int. Cl C04b 33/32, C04b 35/64 |5(1| Field of Search 264/56, 57,

Primary Examiner-Julius Frome Assistant Examiner-John H. Miller Attorneys-Samuel M. Mims, Jr., James 0. Dixon, Andrew M. Hassell, Harold Levine, Melvin Sharp, William E. Hiller and John E. Vandigriff ABSTRACT: The method of sintering amorphous of amorphous metal oxide, and in a preferred embodiment crucibles and tubing of fused silicon dioxide, by depositing a porous layer of amorphous metal oxide on a deposition surface, and thereafter partially sintering the edges of the metal oxide article to less than 90 percent of its theoretical density and substantially sintering the remainder of the metal oxide article to greater than 90 percent of its theoretical density. By only partially sintering the edge portions sufficient frictional forces are produced with the mandrel to exceed the surface tension forces created in the remainder of the metal oxide article by the higher sintering temperature thus preventing longitudinal contraction and crawling encountered in the prior art processes. For a silicon dioxide article, the edges of the mandrel are sintered at a temperature in the range of 1,250C. to 1,350 C.; the remainder of the article is sintered at a temperature in the range of 1,450 C. to 1,550 C, The partially sintered portion is then severed from the fully sintered portion to produce the desired article.

PATENTEDuuv 9 i971 3.619.440

INVENTOR: FOSTER L. GRAY PREVENTION OF CRAWLING OF METAL OXIDE HOLLOW ARTICLES ALONG THE SUPPORT MANDREL DURING SINTERING This invention relates to the production of metal oxide articles and, more particularly, to a technique for densifying and fusing articles of an amorphous metal oxide.

Among the known methods for producingmetal oxide articles, and especially silicon dioxide articles, is one by which a porous layer of an amorphous silicon dioxide is deposited on a surface by vapor phase hydrolysis of silicon tetrachloride and then sintered or densified. A method and apparatus for performing this method are disclosed in a copending application to Herbert J. Moltzan, Ser. No. 744,153, filed July 11, 1968 (TI-3326). In this method, vaporous silicon tetrachloride is combined with a hydrogen and oxygen combustible gas stream, then ignited. The silicon tetrachloride is hydrolyzed to silicon dioxide and deposited on a deposition surface, usually a mandrel at least a part of which has a cylindrical configuration. To completely cover the surface of the mandrel, the mandrel is rotated and translated until a silicon dioxide article is formed.

The article thus formed comprises a porous layer of amorphous silicon dioxide. To be usable, the article must be sintered and densified. When the porous silicon dioxide article is inserted in a sintering furnace and elevated to a temperature of about l,500 C., the silicon dioxide becomes a very viscous liquid and densifies to nearly its theoretical density. However, when the article becomes a viscous liquid, the surface tension forces on the article will cause it to contract and form a droplet or mass appearing much like that of a drop of water on a clean glass surface. These forces must be resisted in some manner. Various techniques are available in the art, among which is the use of a stepped deposition and sintering mandrel of the type disclosed in a copending application to R. Bruce Biddulph, Ser. No. 799,891, filed Feb. 17, 1969 (Tl-3327).

These prior art methods, including the use of stepped mandrels have disadvantages. For example, when a mandrel of cylindrical configuration with end portions of abruptly reduced diameter of one-quarter of an inch are utilized to make silica tubing, the layer of silicon dioxide is deposited over the mandrel surface, including the stepped end portions. The mandrel with its deposit is then placed in a sintering furnace where the temperature is raised to about 1,500 C. The end portions of the silicon dioxide article tend to shrink over the stepped end portions of the mandrel and thus resist the tensile forces tending to contract the article. The drawback of this method, however, is that as the very viscous silica contracts over the stepped portion, it becomes very thin and often breaks, hence defeating the purpose of the stepped portion.

It would be, therefore, desirable to possess a technique for sintering and densifying a metal oxide article, specifically a silicon dioxide article, to eliminate the problems associated with present densification methods. It is desirable to eliminate the tendency of the silicon dioxide to break away from a'stepped mandrel. It is also desirable to develop a technique whereby a cylindrical unstepped mandrel can be utilized.

To overcome the disadvantages of the prior art and to provide a novel technique for sintering metal oxide articles, this invention provides a method for manufacturing a metal oxide article which tends to contract when it is heated to a temperature sufficiently high to fuse and densify the metal oxide, which method comprises depositing a porous layer of amorphous metal oxide on a deposition mandrel, thereafter partially densifying the edge portion of the article while substantially fully densifying the remainder of the article.

In a preferred embodiment, a silicon dioxide article is produced and selectively densified by heating the edge portion of the article to a temperature in the range of 1,250" to 1,350 C., and heating the remainder of the article to a temperature in the range of l,450 to l,550 C.

The invention will be more easily understood by reference to the attached drawings wherein:

FIG. 1 is a cross-sectional partially schematic representation of a vapor phase hydrolysis torch depositing a metal oxid on a stepped mandrel to form a tubular shape;

FIG. 2 is a cross-sectional partially schematic representation of a vapor phase hydrolysis torch depositing a metal oxide on a stepped mandrel to form a crucible shape;

FIG. 3 is a cross-sectional partially schematic representation of the formation of a tubular metal oxide article on an unstepped mandrel;

FIG. 4 is a cross-sectional view of a metal oxide article sintered or densified by a method of the prior art;

FIG. 5 is a cross-sectional view of another article sintered by a method of the prior art;

FIG. 6 is a cross-sectional partially schematic representation of a tubular metal oxide structure after it has been selectively and discriminately sintered on a stepped mandrel;

FIG. 7 is a cross-sectional partially schematic representation of a tubular metal oxide article after it has been selectively and discriminately sintered on an unstepped mandrel;

FIG. 8 is a cross-sectional partially schematic representation of an apparatus for selectively sintering a metal oxide crucible on an unstepped mandrel;

FIG. 9 is a cross-sectional partially schematic representation of a method for removing or severing a selectively sintered crucible from an unstepped mandrel.

The present invention will be described in relation to a preferred embodiment for producing a densified, amorphous silicon dioxide or fused silica article. Although the description will proceed in this manner, it will be understood that the method of this invention is applicable to all metal oxides which will fuse and which will form a very viscous, amorphous liquid having a tendency to contract when being sintered. For example, silicon dioxide at 1,500 C. has a viscosity on the order of 10 to 10 poises. The above metal oxides can be selected from the oxides of those metals appearing in groups A, IIIA, IVA, IIIB, IVB and VB of the Periodic Table as it appears on the flyleaf of Perrys Chemical Engineers Handbook, edited by R. H. Perry, C. H. Chilton and S. D. Kirkpatrick, McGraw Hill Book Co., Inc., New York, 1963.

Referring now to FIG. 1, vaporous silicon tetrachloride is fed from a suitable source into conduit 10 of a vapor phase hydrolysis torch, generally designated as 12. Elemental hydrogen is fed into the torch 12 at 14 and elemental oxygen is fed into the torch at 16. The oxygen and hydrogen are mixed in the torch and ejected in a stream 18 surrounding a stream of vaporous silicon tetrachloride 20. The streams are combined and ignited at 22 causing vapor phase hydrolysis of the silicon tetrachloride to silicon dioxide. The flame is directed toward a deposition surface, initially mandrel 24, depositing a layer of silicon dioxide thereon. The mandrel is rotated and translated in the direction shown by the arrows, thus producing a porous layer of amorphous silicon dioxide on the mandrel 24. The mandrel 24 in this embodiment is of cylindrical configuration; the silicon dioxide article 26 is consequently of cylindrical configuration. The mandrel 24 is shown as having a stepped end portion 28. The end portion 28 has a smaller diameter than that of the main portion 30 of the mandrel 24.

Likewise, in FIG. 2, a porous, amorphous silicon dioxide article is produced by vapor phase hydrolysis of silicon tetrachloride. In this second embodiment a mandrel, about which is formed a crucible shaped silicon dioxide article, is utilized. The mandrel 40 has a generally cylindrical configuration, one end of which is smoothly rounded to fonn a curved bottom article 42, i.e., the bottom portion has generally the shape of a segment of a spheroid. The other end has a stepped portion 44 of smaller diameter than that of the main portion of the mandrel 40.

face can be used in accordance with the method of the presentinvention.

FIGS. 4 and 5 are devoted to illustrating the problems of the prior art. When fused silica, for example, is sintered and densified at a temperature of about l,500 C. on a stepped mandrel 60, the silica article 62 densities or shrinks around the mandrel 60. At that temperature silicon dioxide is a very viscous liquid, the surface tension forces of which cause it to contract no only in a radial direction but also longitudinally in the direction of arrows 64. The stepped portions 66 assist in preventing this longitudinal contraction. However, the fused silica article becomes very thin at edges 68. Frequently, the article will not have sufficient tensile strength to withstand breaking at the edges 68. when this happens the silica article will continue contracting until it forms a toroidally shaped mass 70 as shown in FIG. 5. when an unstepped mandrel is utilized, a phenomenon similar to that illustrated in FIG. 5 will occur.

To alleviate these problems of the prior art, the porous article formed as in FIGS. 1, 2 and 3 is selectively or discriminately sintered. Referring now to FIG. 6, the metal oxide article 76 is partially sintered in zone B and is fully or substantially fully sintered in zone A. The article is partially sintered or densified in zone B at a temperature which will partially densify the structure but will allow it to remain substantially a solid. Thus, substantially no longitudinal viscous flow of the article will occur in zone B. As shown in this figure, a stepped mandrel is utilized to practice the invention. The partially sintered solid portion 78 in zone B locks onto the stepped portion of the mandrel thus preventing longitudinal flow, while the remainder of the article in zone A is fully sintered to nearly its theoretical density.

Similarly, in FIG. 7, the method of this invention is utilized in conjunction with a substantially cylindrical unstepped mandrel 80. The metal oxide article 82 is partially sintered in zone B and is fully sintered in zone A. In a preferred embodiment, a silicon dioxide article of, for example, a tubular configuration as shown in FIG. 7 is partially sintered in zone B at temperatures ranging from about l,250 to l,350 C. The remainder of the article is substantially fully sintered at temperatures in the range of from about l,450 to 1,550 C. At these temperatures the silicon dioxide article is partially sintered to less than 90 percent of its theoretical density, and preferably to less than 85 percent of its theoretical density. Also, at these temperatures, the article is fully densitied to greater than 90 percent of its theoretical density, and preferably to greater than 99 percent of its theoretical density. In the embodiment show in FIG. 7, the frictional forces between the mandrel 80 and the partially sintered article 82 exceed the surface tension forces tending to cause longitudinal contraction, thus preventing any such contraction of the metal oxide article.

An apparatus for carrying out the method of this invention is illustrated in FIG. 8. A mandrel 90 with its crucible-shaped coating or layer 92 of metal oxide is positioned on a support 94. A bell jar 96 and a suitable heat shield 98 are positioned over the base 94. The area under bell jar 96 is evacuated via conduit 100 and a suitable vacuum pump (not shown).

An induction heating coil 102 is positioned around shield 98 and is constructed so that the energy input into zone A is greater than the energy input to zone B. A susceptor 95 is constructed of a conductive material, normally and preferably graphite, ad surrounds the metal oxide article 92. Preferably a radiofrequency energy source is utilized to closely control the energy input to the susceptor 95 through out zone A. The metal oxide article 92 can be fully sintered in zone A. By controlling the energy input to zone B at a level lower than that to zone A, the metal oxide can be only partially sintered in zone B. Induction heating is well known and various methods for selectively controlling its energy input to a susceptor are also well known. For example, the coil spacing in zone B can be greater than that in zone A to achieve lower energy input to zone B.

After the metal oxide article is selectively or discriminately sintered and is cooled down to room temperature, the partially sintered portion of the article is severed from the fully sintered portion of the article as illustrated in FIG. 9. In FIG. 9, the

mandrel 108 is rotated in the direction shown by arrow 106. A saw 110, preferably of circular configuration and having a cutting edge sufficiently hard to penetrate the metal oxide article 112, is rotated in the direction-of the arrow 114. The saw and mandrel, rotating on parallel axes are brought together so that the partially sintered portion I16 of the article I12 is severed from the main portion of the article 1112 along a plane I 18. In this figure a crucible shaped article has been shown. It is to be readily understood, however, that if an article such as that shown in FIG. 6 is produced, both end portions 78 will be severed from the main body portion of the tubular-shaped article 76.

To better explain the procedure of this invention, an exemplification of a preferred mode of carrying it out follows.

A porous layer 2 inches thick of amorphous silicon dioxide is deposited on a stepped mandrel by vapor phase hydrolysis of silicon tetrachloride in the manner described in conjunction with FIG. 1. The mandrel is 14 inches long, cylindrical in shape, and has a diameter through its major portion of 3 inches. The stepped end portions of the mandrel are cylindrically shaped having a diameter of 3 7/16 inches and a height or length of three-quarters of an inch. The mandrel and deposit are removed from the deposition enclosure and inserted in a sintering furnace. The density of the oxide layer prior to sintering is in the range of 8 to 15 percent of the theoretical density of silicon dioxide.

A radiofrequency induction heater is utilized to bring the temperature of the mandrel and oxide layer from the ends thereof to points 2 inches longitudinally inward up to about l,300 C. The remaining lO-inch center portion is maintained at a temperature of about l,500 C. After 30 minutes at the peak temperature of l,500 C., the mandrel is removed from the sintering furnace and cooled. The density of the middle 10 inches of the silicon dioxide article is greater than 99.5 percent of theoretical. The average density of the 2-inch end portions is about percent of theoretical. The silica article conforms to a shape very similar to that illustrated in FIG. 6. The fully densified portion is then severed from the partially densified end portions by the procedure described in conjunction with FIG. 9.

The above procedure is repeated except that the entire silicon dioxide article is sintered at a temperature of 1,500 C. for 30 minutes. The silicon dioxide article pulls away from the stepped end portions of the mandrel and forms a toroidally shaped article having a density greater than 99 percent of theoretical. For practical purposes, the article is unusable.

Although the foregoing invention has been described in conjunction with a preferred embodiment, it is to be understood that various modifications of this invention can be made without departing from the spirit of the invention. The scope of'the invention is intended to be limited only by the appended claims.

What is claimed is:

I. In the method of making a fused metal oxide article wherein the oxide is selected from the metals of groups IIA, IIIA, IVA, lIIB, IVE, and VB of the Periodic Table which form an oxide and which upon reaching a temperature sufficient to fuse the oxide tend to contract comprising the steps of;

a. Depositing a porous layer of the amorphous oxide onto a refractory mandrel by vapor phase hydrolysis of the metal halide in the presence of an ignited combustible gas stream;

b. sintering the porous layer while on the mandrel to fuse and densify the oxide layer to near its theoretical density; wherein the improvement comprises; heating the edge portions of the oxide layer to a first temperature while heating the remainder of the oxide layer to a second temperature higher than said first temperature and sufficient to cause it to fuse and densify to near theoretical density, said first temperature being controlled so that the edge portions only partially densify and maintain sufficient frictional forces with the mandrel to prevent longitudinal contraction of the metal oxide layer.

2. The method of claim 1 wherein the metal oxide is silicon dioxide.

3. The method of claim 3 wherein a porous layer in the shape of a cylinder is deposited, the method including partially densifying the end portions of the cylindrical layer and substantially fully densifying the portion intermediate the ends to an extent greater than the end portions.

4. The method of claim 3 wherein the end portions are densified to less than 85 percent of their theoretical density.

5. The method of claim 4 wherein the intermediate portion is densified to greater than 90 percent of its theoretical density.

6. The method of claim 4 wherein the intermediate portion is densified to greater than 99 percent of its theoretical density.

7. The method of claim 1 wherein the metal halide is silicon tetrachloride and is hydrolyzed to silicon dioxide.

8. The method of claim 7 for manufacturing a fused silicon dioxide article wherein the edge portion of the article is partially densified at a temperature of from 1,250 to l,350 C. and the remainder of the article is densified at a temperature of from l,450 to l,550 C.

9. The method of claim 8 including the step of severing the partially densified portion of the article from the fully densified portion of the article.

10. The method of claim 8 wherein the edge portion of the article is densified to less than 90 percent of its theoretical density.

11. The method of claim 10 wherein the edge portion is densified to less than percent of its theoretical density.

12. The method of claim 3 wherein the remainder of the article is densified to greater than percent of its theoretical density.

13. The method of claim 12 wherein the remainder of the article is densified to greater than 99 percent of its theoretical density.

14. The method of claim 8 wherein the porous silicon dioxide layer is deposited on a mandrel having generally a cylindrical configuration, the end portions of which have an abruptly reduced diameter to form a mandrel having stepped end portions.

15. The method of claim 14 wherein the edge portion of the silicon dioxide article adjacent the stepped portion of the mandrel is partially sintered and the portion of the article intermediate the stepped portions of the mandrel is fully sintered.

16. The method of claim 8 wherein the porous silicon dioxide layer is deposited on a mandrel having a generally cylindrical configuration, one end portion of which has an abruptly reduced diameter to form a stepped portion, the other end of which is smoothly rounded having generally the shape of a segment of a spheroid.

17. The method of claim 11 wherein the portion of the article so formed adjacent the stepped portion is partially sintered and the remainder of the article is sintered. 

2. The method of claim 1 wherein the metal oxide is silicon dioxide.
 3. The method of claim 3 wherein a porous layer in the shape of a cylinder is deposited, the method including partially densifying the end portions of the cylindrical layer and substantially fully densifying the portion intermediate the ends to an extent greater than the end portions.
 4. The method of claim 3 wherein the end portions are densified to less than 85 percent of their theoretical density.
 5. The method of claim 4 wherein the intermediate portion is densified to greater than 90 percent of its theoretical density.
 6. The method of claim 4 wherein the intermediate portion is densified to greater than 99 percent of its theoretical density.
 7. The method of claim 1 wherein the metal halide is silicon tetrachloride and is hydrolyzed to silicon dioxide.
 8. The method of claim 7 for manufacturing a fused silicon dioxide article wherein the edge portion of the article is partially densified at a temperature of from 1,250* to 1,350* C. and the remainder of the article is densified at a temperature of from 1,450* to 1,550* C.
 9. The method of claim 8 including the step of severing the partially densified portion of the article from the fully densified portion of the article.
 10. The method of claim 8 wherein the edge portion of the article is densified to less than 90 percent of its theoretical density.
 11. The method of claim 10 wherein the edge portion is densified to less than 85 percent of its theoretical density.
 12. The method of claim 8 wherein the remainder of the article is densified to greater than 90 percent of its theoretical density.
 13. The method of claim 12 wherein the remainder of the article is densified to greater than 99 percent of its theoretical density.
 14. The method of claim 8 wherein the porous silicon dioxide layer is deposited on a mandrel having generally a cylindrical configuration, the end portions of which have an abruptly reduced diameter to form a mandrel having stepped end portions.
 15. The method of claim 14 wherein the edge portion of the silicon dioxide article adjacent the stepped portion of the mandrel is partially sintered and the portion of the article intermediate the stepped portions of the mandrel is fully sintered.
 16. The method of claim 8 wherein the porous silicon dioxide layer is deposited on a mandrel having a generally cylindrical configuration, one end portion of which has an abruptly reduced diameter to form a stepped portion, the other end of which is smoothly rounded having generally the shape of a segment of a spheroid.
 17. The method of claim 11 wherein the portion of the article so formed adjacent the stepped portion is partially sintered and the remainder of the article is sintered. 